A common use for light weight high strength beams can be found in boom trucks--trucks that are used to lift a cage or similar containing a person and/or machinery to an elevated position. Such trucks may be employed in maintenance of buildings, high voltage wires, telephone wires, and the like, or in attending to trees, especially fruit trees, or a variety of other applications. In these boom trucks, it is important that the boom be as strong as possible so that a maximum amount of weight can be supported at the outer end thereof. It is also important that the boom be as long as possible, or at least as long as required, so that desired elevated positions can be reached.
It is also necessary that the beam be as strong as possible, but also be as light weight as possible in order not to add unnecessarily to the overall weight of the boom, since the beam must also support its own weight.
Such beams typically experience tensile stresses in their upper region and compressive stress in their lower region. Further, when the beams are subject to cantilever bending they also experience shear stresses in their side walls. Torsional load will also create additional shear stresses.
In order to construct a beam that can provide resistance to all these types of stresses, especially with these stresses being fairly high, and also provide a light enough weight beam, a composite material or materials are typically used. Further, such materials are typically formed into a beam by a multi-step method of manufacture.
Another very important characteristic of the beam is that it has an extremely high dielectric strength. The beam must be able to withstand a very high voltage applied thereto while allowing an electrical current that is in the order of a few microamperes to pass. Such an electrical condition can occur if the beam comes in contact with hydro wires. Indeed, it is necessary that the beam be able to withstand and insulate high electrical voltages in order to protect anyone working in a bucket suspended at the end of the boom. It is usual for workmen working on high voltage electrical power lines (in the order of up to several hundred thousand volts) to work on those lines live--that is, the lines are operating while being worked on.
It is therefore necessary that the material or materials used have a high dielectric strength. It is necessary that the beam not have any voids therein, to preclude the trapping of moisture. Having moisture trapped with the beam, whether in voids within the material, or in voids between material parts, could allow for electrical conductance, sufficient enough to make the beam unsafe.
The above mentioned properties of the beam are necessary in order for a boom truck using such a beam to be safe. The ultimate safety of the boom truck is also dependent on proper installation of the beam therein and subsequent safe use, so that the above mentioned properties of the beam are not comprised.